Method for regulating an output voltage using a converter configured to operate in a critical conduction mode and a frequency fold-back mode and structure

ABSTRACT

In accordance with an embodiment, a converter includes a power factor controller that varies the switching frequency of a switching transistor in accordance with a signal representative of power at the input of the converter.

The present application is a Division application of U.S. patent application Ser. No. 13/990,884 filed on Aug. 19, 2013, by Joel Turchi, titled “METHOD FOR REGULATING AN OUTPUT VOLTAGE USING A CONVERTER CONFIGURED TO OPERATE IN A CRITICAL CONDUCTION MODE AND A FREQUENCY FOLD-BACK MODE” which is a nonprovisional application of a 371 international application having Prior Application Number PCT/US2010/062105 and a filing date of Dec. 24, 2010, which continuation application and 371 international application are hereby incorporated by reference in their entirety, and priority thereto for common subject matter is hereby claimed.

BACKGROUND

The present invention relates, in general, to electronics and, more particularly, to methods of forming semiconductor devices and structure.

Power converter circuits are typically used within power supply applications and may include DC to DC converters, AC to DC converters, DC to AC converters, and AC to AC converters. These types of circuits may include rectification elements, energy storage elements, switching elements, etc. For example, in ac/dc rectification applications, a converter system may include a full wave voltage rectifier, typically in the form of a diode bridge and a main Switch Mode Power Supply (“SMPS”) to provide regulation of the output waveform. PFC circuits are inserted between the line and the main SMPS to draw a sinusoidal current from the line and to provide Direct Current (“DC”) voltage to the main SMPS. A drawback with PFC circuits is that their performance is degraded under a light or low load condition because of large switching losses from a switching transistor, an inductor, or other switching elements. A system that improves the efficiency of the PFC circuit at a low load is disclosed in International Patent Application No. PCT/IB2010/050154 to NXP B.V. having Publication No. WO2010/082172A1 and publication date of Jul. 22, 2010.

Other types of converter circuits having large switching losses include flyback converters, forward converters, etc.

Accordingly, it would be advantageous to have a method and circuit suitable for use as a converter that mitigates switching losses. It would be of further advantage for the method and structure to be cost efficient to implement.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be better understood from a reading of the following detailed description, taken in conjunction with the accompanying drawing figures, in which like reference characters designate like elements and in which:

FIG. 1 is a circuit schematic of a converter that includes a power factor controller in accordance with an embodiment of the present invention;

FIG. 2 is a plot of inductor current versus time for the converter of FIG. 1 in accordance with an embodiment of the present invention;

FIG. 3 is a timing diagram of circuit parameters of the converter of FIG. 1 in accordance with an embodiment of the present invention;

FIG. 4 is a timing diagram of circuit parameters of the converter of FIG. 1 in accordance with an embodiment of the present invention;

FIG. 5 is a timing diagram of circuit parameters of the converter of FIG. 1 in accordance with an embodiment of the present invention;

FIG. 6 is a timing diagram of circuit parameters of the converter of FIG. 1 in accordance with an embodiment of the present invention;

FIG. 7 is a timing diagram of circuit parameters of the converter of FIG. 1 in accordance with an embodiment of the present invention;

FIG. 8 is a timing diagram of circuit parameters of the converter of FIG. 1 operating in critical conduction mode in accordance with an embodiment of the present invention;

FIG. 9 is a timing diagram of circuit parameters of the converter of FIG. 1 operating in frequency fold-back mode in accordance with an embodiment of the present invention;

FIG. 10 is a timing diagram of circuit parameters of the converter of FIG. 1 operating in frequency fold-back mode in accordance with an embodiment of the present invention;

FIG. 11 is a circuit schematic of the power factor controller of FIG. 1 in accordance with an embodiment of the present invention;

FIG. 12 is a circuit schematic of a regulator stage of the power factor controller of FIG. 11 in accordance with an embodiment of the present invention;

FIG. 13 is a circuit schematic of a control stage of the power factor controller of FIG. 11 in accordance with an embodiment of the present invention; and

FIG. 14 is a circuit schematic of a current information generation stage of the power factor controller of FIG. 11 in accordance with an embodiment of the present invention.

For simplicity and clarity of illustration, elements in the figures are not necessarily to scale, and the same reference characters in different figures denote the same elements. Additionally, descriptions and details of well-known steps and elements are omitted for simplicity of the description. As used herein current carrying electrode means an element of a device that carries current through the device such as a source or a drain of an MOS transistor or an emitter or a collector of a bipolar transistor or a cathode or anode of a diode, and a control electrode means an element of the device that controls current flow through the device such as a gate of an MOS transistor or a base of a bipolar transistor. Although the devices are explained herein as certain N-channel or P-channel devices, or certain N-type or P-type doped regions, a person of ordinary skill in the art will appreciate that complementary devices are also possible in accordance with embodiments of the present invention. It will be appreciated by those skilled in the art that the words during, while, and when as used herein are not exact terms that mean an action takes place instantly upon an initiating action but that there may be some small but reasonable delay, such as a propagation delay, between the reaction that is initiated by the initial action. The use of the word approximately, about, or substantially means that a value of an element has a parameter that is expected to be very close to a stated value or position. However, as is well known in the art there are always minor variances that prevent the values or positions from being exactly as stated. It is well established in the art that variances of up to about ten percent (10%) (and up to twenty percent (20%) for semiconductor doping concentrations) are regarded as reasonable variances from the ideal goal of being exactly as described.

It should be noted that a logic zero voltage level (V_(L)) is also referred to as a logic low voltage and that the voltage level of a logic zero voltage is a function of the power supply voltage and the type of logic family. For example, in a Complementary Metal Oxide Semiconductor (CMOS) logic family a logic zero voltage may be thirty percent of the power supply voltage level. In a five volt Translator-Translator Logic (TTL) system a logic low voltage level may be about 0.8 volts, whereas for a five volt CMOS system, the logic zero voltage level may be about 1.5 volts. A logic one voltage level (V_(H)) is also referred to as a logic high voltage level and, like the logic zero voltage level, the logic high voltage level also may be a function of the power supply and the type of logic family. For example, in a CMOS system a logic one voltage may be about seventy percent of the power supply voltage level. In a five volt TTL system a logic one voltage may be about 2.4 volts, whereas for a five volt CMOS system, the logic one voltage may be about 3.5 volts.

DETAILED DESCRIPTION

Generally the present invention provides a power factor controller and a method for mitigating switching losses in a converter circuit. In accordance with an embodiment of the present invention, a power factor controller includes a regulator stage, a control stage, and an output stage. The regulator stage generates a regulated electrical signal V_(REGUL) that is transmitted to an input of the output stage and the control stage generates a clock signal V_(CLK) that is transmitted to another input of the output stage. In response to signals V_(REGUL) and V_(CLK), the output stage generates a drive signal V_(DRV) suitable for driving a switching transistor under light, medium, and heavy loads that mitigates switching losses of the switching transistor.

In accordance with another embodiment of the present invention, a method for operating the power factor controller and converter is provided. In response to a high input current, a converter 10 operates in critical conduction mode or in a quasi-resonant (QR) mode, and in response to the input current being low, i.e., in a light load condition, or near the zero crossing, converter 10 operates in a frequency fold-back mode and a delay or dead-time is introduced before the start of the next current cycle, wherein the delay is set by a timer in accordance with a magnitude of the input current and controls the dead time rather than the switching period or the total off time. Hence variations in the current cycle duration do not affect the dead time.

In accordance with other embodiments of the present invention and in response to operating in the frequency fold-back mode, the next current cycle, i.e., the next gate drive signal, is started in response to a falling edge of the drain-to-source voltage V_(DS) of the switching transistor. More particularly, the next current cycle is started in response to a falling edge of an oscillatory portion of the drain-to-source voltage V_(DS), thus the gate drive signal is synchronized to the drain-to-source voltage V_(DS), and more particularly to a valley of the drain-to-source voltage V_(DS). It should be noted that at the end of an oscillator period the drain-to-source voltage of the switching transistor may not be at a valley, but can be at a higher voltage. Synchronizing the start of the gate drive signal to the valley reduces the switching losses.

In accordance with other embodiments of the present invention, in which the input current is very small, the converter can enter a skip mode in which the skipping is synchronized to the input current and skips at the line zero crossing.

In accordance with other embodiments of the present invention and in response to operating in the frequency fold-back operating mode, a delay or dead-time is introduced before the start of the next current cycle, which is started in response to a valley of the drain-to-source voltage V_(DS). It should be noted that the delay is set by a timer in accordance with a magnitude of the input current.

FIG. 1 is a block diagram of a converter 10 in accordance with an embodiment of the present invention. Converter 10 includes a switching device 12 having a control electrode configured to receive a drive signal (V_(DR)) from a power factor controller 14 and a current carrying electrode coupled to an input stage 16 through an inductor 18A and coupled to a load 20 through a diode 36. Power factor controller 14 may be referred to as a drive stage, a driver stage, or a drive circuit. More particularly, switching device 12 may be a power Field Effect Transistor (FET) having a drain electrode, a source electrode, and a gate electrode. Input stage 16 may be comprised of an Electromagnetic Interference (EMI) filter connected to a rectifier circuit 24, comprising diodes 24A, 24B, 24C, and 24D. Input terminals 23 and 25 of EMI filter 22 may be coupled for receiving an AC line signal. The anodes of diodes 24B and 24D are commonly connected together and for receiving a source of operating potential such as for example a potential V_(SS). The cathode of diode 24B is commonly connected to an output terminal 22A of EMI filter 22 and to the anode of diode 24A and the cathode of diode 24D is commonly connected to an output terminal 22B of EMI filter 22 and to the anode of diode 24C. The cathodes of diodes 24A and 24C are commonly connected together and serve as an output 17 of input stage 16. A capacitor 26 is coupled between output 17 and the commonly connected anodes of diodes 24B and 24D.

Inductor 18A is magnetically coupled to an inductor 18B to form a magnetic element 18. Inductor 18A has a terminal connected to output 17 of input stage 16 and a terminal connected to the drain electrode of power FET 12. Inductor 18B has a terminal coupled for receiving a source of operating potential such as, for example, operating potential V_(SS) and a terminal connected to an input pin 14F of power factor controller 14 through a diode 27 and a resistor 28. The voltage across inductor or winding 18B is representative of the inductor phase, i.e., charging, demagnetization, or dead-time phase. More particularly, the terminal of inductor 18B is connected to the anode of diode 27 and the cathode of diode 27 is commonly connected to terminals of resistors 28 and 30. The other terminal of resistor 28 is connected to input pin 14F of power factor controller 14 and the other terminal of resistor 30 is coupled for receiving source of operating potential V_(SS). Inductor 18B, diode 27, and resistors 28 and 30 cooperate to form a zero current detection circuit 19. Alternatively, a resistor can be inserted between inductor 18A and input pin 14F. It should be noted that the method for performing zero current detection is not limited to magnetically coupling two inductors together and coupling a diode and a pair of resistors to one of the inductors. For example, zero current detection may be accomplished by inserting a single resistor between the node common to the anodes of diodes 24B and 24D and the terminal of capacitor 26 and source of operating potential V_(SS), and detecting zero current through the resistor.

The gate electrode of power FET 12 is connected to output terminal 14E of power factor controller 14 and the source electrode of power FET 12 is coupled for receiving source of operating potential V_(SS) through a resistor 32.

An input/output terminal 14D of power factor controller 14 is coupled for receiving source of operating potential V_(SS) through a resistor 34. In addition, power factor controller 14 has input terminals 14A, 14B, and 14C which are further described with reference to FIG. 11. Input 14A may be coupled to a resistor divider network 13 that comprises resistors 13A and 13B, where a terminal of resistor 13A and a terminal of resistor 13B are commonly connected to input terminal 14A, the other terminal of resistor 13A is coupled for receiving the potential at node 40, i.e., potential V_(OUT) of the power factor correction stage, and the other terminal of resistor 13B is coupled for receiving a source of operating potential such as, for example, potential V_(SS). Input 14C may be coupled to a resistor divider network 15 that comprises resistors 15A and 15B, where a terminal of resistor 15A and a terminal of resistor 15B are commonly connected to input terminal 14C, the other terminal of resistor 15A is coupled for receiving an input signal V_(IN), and the other terminal of resistor 15B is coupled for receiving a source of operating potential such as, for example, potential V_(SS). A capacitor 11 has a terminal connected to input terminal 14B and the other terminal of capacitor 11 is coupled for receiving a source of operating potential such as, for example, potential V_(SS).

The drain terminal of power FET 12 is connected to an anode of diode 36. A capacitor 38 is connected between the cathode of diode 36 and source of operating potential V_(SS) and load 20 is connected between the cathode of diode 36 and source of operating potential V_(SS). It should be noted that the commonly connected terminals of capacitor 38 and load 20 and the cathode of diode 36 may form an output node 40 at which an output signal V_(OUT) appears.

In operation, converter 10 receives an input signal at inputs 23 and 25, rectifies the input signal and, in response to the switching activity of switching transistor 12 and the rectified input signal, generates an input current I_(IN). It should be noted that input current I_(IN) is an average value of inductor current I_(18A) over a switching period. As the load becomes lighter, the switching losses of switching transistor 12 degrade the efficiency of converter 10. Converter 10 is configured to operate in a Current Controlled Frequency Fold-back (CCFF) fashion, where converter 10 operates in Critical Conduction Mode (CrM) in response to a large current drawn from the line and in frequency fold-back mode in response to a lighter current or less current drawn from the line. The large current is referred to as a heavy current or heavy input current and the lighter current is referred to as a light current or a light input current.

FIG. 2 is a timing diagram 42 of inductor current I_(18A) in response to a critical conduction mode operation (plot 43 of timing diagram 42) and frequency fold-back mode of operation (plot 44 of timing diagram 42). What is shown in FIG. 2 is a timing diagram or plot 43 of the inductor current, I_(18A), for a converter 10 operating in the critical current conduction mode, i.e., under a heavy input current, and a timing diagram or plot 44 in response to converter 10 operating in the frequency fold-back mode of operation, i.e., under medium or light input current. In the critical conduction mode shown in timing diagram 43, inductor current I_(18A) increases linearly from time t₀ to time t₁ in response to a drive signal V_(DR) at the gate of switching transistor 12 (shown in FIG. 1) being at a logic high voltage level, i.e., switching transistor 12 is on. Inductor current I_(18A) transitions from a zero current level at time t₀ to a peak current level I_(pk) at time t₁. In response to switching transistor 12 turning off at time t₁, inductor current I_(18A) decreases linearly from peak current level I_(pk) at time t₁ to zero at time t₂. At time t₂, inductor current I_(18A) is substantially zero and the next current cycle begins. The time from time t₀ to time t₁ is referred to as the on-time t_(on) because switching transistor 12 is “on” and the time from time t₁ to time t₂ may be referred to as the off time t_(off) because switching transistor 12 is “off.”

In response to a light input current or a medium input current condition, converter 10 operates in a frequency fold-back mode. Inductor current I_(18A) for the frequency fold-back mode is illustrated in timing diagram 44. In accordance with the frequency fold-back operating mode, inductor current I_(18A) increases linearly from zero at time t₀ to peak current I_(pk) at time t₁ in response to drive signal V_(DR) at the gate of switching transistor 12. In response to switching transistor 12 turning off at time t₁, inductor current I_(18A) decreases linearly from current level I_(pk) at time t₁ to zero at time t₂. In response to a control signal from power factor controller 14, switching transistor 12 remains off from time t₂ to time t₃. The time from time t₀ to time t₁ may be referred to as the on-time t_(on) because switching transistor 12 is on; the time from time t₁ to time t₂ may be referred to as the off time t_(off) because switching transistor 12 is off but inductor current I_(18A) is flowing; and the time from time t₂ to time t₃ may referred to as the dead-time because switching transistor 12 is off and inductor current I_(18A) is zero. At time t₃, power controller 14 generates a drive signal V_(DR) that turns on switching transistor 12 thereby restarting the on-time t_(on).

FIG. 3 is a timing diagram 45 that further illustrates signals used by converter 10 to generate drive signal V_(DR) which may turn on power transistor 12. What is shown in FIG. 3 is a plot 46 of inductor current I_(18A) over a time period from time t₀ to time t₄. Inductor current I_(18A) over this time period has been described with reference to FIG. 2. In addition, FIG. 3 illustrates a current information signal V_(FF) that is a signal value representative of the power at the input of converter 10. By way of example, information signal V_(FF) is a voltage representative of input current I_(IN). FIG. 3 also illustrates a preset reference voltage V_(REFM) that is a user defined value. In response to current information signal V_(FF) being greater than preset reference voltage V_(REFM), converter 10 operates in a critical conduction mode and in response to current information signal V_(FF) being less than preset reference voltage V_(REFM), converter 10 operates in a frequency fold-back mode. Accordingly, FIG. 3 illustrates operation in the frequency fold-back mode. It should be noted that current information signal V_(FF) is generated by coupling resistor 34 (shown in FIG. 1) to input/output terminal 14D of power factor controller 14 and injecting a current representative of input current I_(IN) into resistor 34. Alternatively, current I_(IN) can be directly sensed and used to develop current information signal V_(FF). Restart signal V_(SUM) is developed internally by power factor controller 14 and is the sum of current information signal V_(FF) and a ramp signal generated by an output stage 100 of power factor controller 14. Output stage 100 is described with reference to FIG. 11. Between times t₀ and t₂, the ramp signal is zero, thus current information signal V_(FF) and restart signal V_(SUM) are substantially the same as indicated by plots 47 and 48. At time t₂, output stage 100 generates a ramp signal that is summed with or added to current information voltage V_(FF). Thus, restart signal V_(SUM) increases during the time period from time t₂ to time t₃. At time t₃, restart signal V_(SUM) substantially equals preset reference signal V_(REFM). In response to start signal V_(SUM) being substantially equal to preset reference signal V_(REFM), power factor controller 14 generates a drive signal V_(DR) that turns on switching transistor 12. Accordingly, at time t₃, inductor current I_(18A) begins to increase.

FIG. 4 is a timing diagram 45A that further illustrates signals used by converter 10 to generate drive signal V_(DR) which may turn on power transistor 12. Timing diagram 45A is similar to timing diagram 45 except that it illustrates an embodiment in which the ramp generated by output stage 100 has a negative slope. Like in timing diagram 45, restart signal V_(SUM) is developed internally by power factor controller 14 but it is the difference between preset reference voltage V_(REFM) and a ramp signal generated by an output stage 100 of power factor controller 14. Between times t₀ and t₂, the ramp signal is zero and preset reference voltage V_(REFM) is substantially equal to restart signal V_(SUM) as indicated by plots 47A and 49A. At time t₂, output stage 100 generates a ramp signal that is subtracted from preset reference voltage V_(REFM). Thus, restart signal V_(SUM) decreases during the time period from time t₂ to time t₃. At time t₃, restart signal V_(SUM) substantially equals current information signal V_(FF). In response to start signal V_(SUM) being substantially equal to current information signal V_(FF), power factor controller 14 generates a drive signal V_(DR) that turns on switching transistor 12. Accordingly, at time t₃, inductor current I_(18A) begins to increase.

FIG. 5 is a timing diagram 51 that illustrates drain-to-source voltage V_(DS) of switching transistor 12 operating in critical conduction mode in response to switching transistor 12 turning off and turning on. More particularly, timing diagram 51 includes a plot 51A illustrating drain-to-source voltage V_(DS) in response to a gate drive signal shown in plot 51B. At time t₀, the gate drive signal transitions from a logic high voltage level (V_(H)) to a logic low voltage level (V_(L)). In response to the gate drive signal transitioning to logic low voltage level V_(L), drain-to-source voltage V_(DS) begins to transition from voltage level V_(ON) to voltage level V_(OFF). At time t₁, drain-to-source voltage V_(DS) reaches voltage level V_(OFF). At time t₂, drain-to-source voltage V_(DS) begins to decrease in response to leakage currents discharging the parasitic drain-to-source capacitance of switching transistor 12. It should be noted that at time t₂, inductor current I_(18A) is substantially zero. (See time t₂ in plot 43 of FIG. 2). However, there may be a delay in asserting the gate drive voltage so that the drain-to-source voltage V_(DS) is at a valley, i.e., becomes a minimum, to reduce switching losses. At time t₃, the gate drive signal transitions from logic low voltage level V_(L) to logic high voltage level V_(H), which turns on switching transistor 12. As discussed with reference to time t₂, there may be a delay before the gate drive signal is asserted. This delay occurs between times time t₂ and time t₃. At time t₄, the gate drive signal transitions from logic high voltage level V_(H) to logic low voltage level V_(L) which turns off switching transistor 12. In response to switching transistor 12 being off, drain-to-source voltage V_(DS) transitions to voltage level V_(OFF). Because converter 10 is operating in the critical conduction mode, there is no delay in generating the gate drive signal.

FIG. 6 is a timing diagram 53 that illustrates drain-to-source voltage V_(DS) of switching transistor 12 in frequency fold-back mode in response to switching transistor 12 turning off and turning on. The boost stage for the converter 10 having timing diagram 53 is operating in a controlled frequency discontinuous operating mode. More particularly, timing diagram 53 includes a plot 53A illustrating drain-to-source voltage V_(DS) in response to a gate drive signal shown in plot 53B. At time t₀, the gate drive signal transitions from logic high voltage level V_(H) to logic low voltage level V_(L). In response to the gate drive signal transitioning to logic low voltage level V_(L), drain-to-source voltage V_(DS) transitions from voltage level V_(ON) to voltage level V_(OFF). At time t₁, drain-to-source voltage signal V_(DS) reaches voltage level V_(OFF). At time t₂, drain-to-source voltage V_(DS) begins to oscillate. The oscillation results from a tank circuit formed by inductor 18A and the parasitic capacitances attached to the drain of switching transistor 12 and the capacitance associated with diode 36. The drain-to-source voltage V_(DS) continues to oscillate between times t₂ and t₇, wherein voltage V_(DS) has relative minima at times t₃, t₅, and t₇ and relative maxima at times t₄ and t₆. The relative voltage minima at times t₃, t₅, and t₇ are referred to as valleys and the time period between times t₂ and t₇ is referred to as a delay period. As will be discussed below, in accordance with embodiments of the present invention, the gate drive signal transitions from logic low voltage level V_(L) to logic high voltage level V_(H) after a last falling edge of drain-to-source voltage V_(DS) during the delay period. At time t₇, the gate drive signal transitions from logic low voltage level V_(L) to logic high voltage level V_(H), which turns on switching transistor 12. Because converter 10 is operating in the frequency fold-back mode, power factor controller 14 delays the transition of the gate drive signal from logic low voltage level V_(L) to logic high voltage level V_(H) in accordance with inductor current I_(18A) and whether drain-to-source voltage V_(DS) is at a valley.

FIG. 7 is a timing diagram 55 that illustrates drain-to-source voltage V_(DS) of switching transistor 12 in frequency fold-back mode in response to switching transistor 12 turning off and turning on. Input current I_(IN) for the drain-to-source voltage V_(DS) shown in timing diagram 55 of FIG. 7 is lower than input current I_(IN) for drain-to-source voltage V_(DS) shown in timing diagram 53 of FIG. 6. Like timing diagram 53, the boost stage for the converter 10 having timing diagram 55 is operating in a controlled frequency discontinuous operating mode. Timing diagram 55 includes a plot 55A illustrating drain-to-source voltage V_(DS) in response to a gate drive signal shown in plot 55B. At time t₀, the gate drive signal transitions from logic high voltage level V_(H) to logic low voltage level V_(L). In response to the gate drive signal transitioning to logic low voltage level V_(L), drain-to-source voltage V_(DS) transitions from voltage level V_(ON) to voltage level V_(OFF). At time t₁, drain-to-source voltage signal V_(DS) reaches voltage level V_(OFF). At time t₂, drain-to-source voltage V_(DS) begins to oscillate because of the tank circuit formed by inductor 18A and the parasitic capacitances associated with the drain of switching transistor 12 and with diode 36. The drain-to-source voltage V_(DS) continues to oscillate during the time period from time t₃ to time t₁₁, wherein voltage V_(DS) has relative minima at times t₃, t₅, t₇, t₉, and t₁₁ and relative maxima at times t₄, t₆, t₈, and t₁₀. The drain-to-source voltage V_(DS) having relative minima at times t₃, t₅, t₇, t₉, and t₁₁ are referred to as valleys and the time period between times t₂ and t₁₁ is referred to as a delay period. As will be discussed below, in accordance with embodiments of the present invention, the gate drive signal transitions from logic low voltage level V_(L) to logic high voltage level V_(H) after a last falling edge of drain-to-source voltage V_(DS) that occurs during the delay period. At time t₁₁, the gate drive signal transitions from logic low voltage level V_(L) to logic high voltage level V_(H), which turns on switching transistor 12. Because converter 10 is operating in the frequency fold-back mode, power factor controller 14 delays the transition of the gate drive signal from logic low voltage level V_(L) to logic high voltage level V_(H) in accordance with input current I_(IN) and whether drain-to-source voltage V_(DS) is at a valley. Because input current I_(IN) of plot 55A is lower than input current I_(IN) of plot 53A shown in FIG. 6, there are more valleys in plot 55A than in plot 53A.

As discussed above, converter 10 is configured to operate in a critical conduction mode in response to current information signal V_(FF) being greater than preset reference voltage V_(REFM) and in a frequency fold-back mode in response to current information signal V_(FF) being less than preset reference voltage V_(REFM). Current information signal V_(FF) is a signal that is representative of the value of input current I_(IN), which is an averaged value of inductor current I_(18A) over a switching period. Thus, power factor controller 14 is configured to select between the operating modes in accordance with the level of current flowing through inductor 18A and to provide a drive or control signal V_(DR) to the gate of switching transistor 12, which operates as a switch. It should be noted that in response to drive signal V_(DR), switching transistor 12 may be configured to be “on” or “off.” In an “off” configuration switching transistor 12 operates as a switch in an open position and in an “on” configuration switching transistor 12 operates as a switch in a closed position. In response to drive signal V_(DR) being at a logic high voltage level, switching transistor 12 operates as a switch in a closed configuration and in response to drive signal V_(DR) being at a logic low voltage level, switching transistor 12 operates as a switch in an open configuration.

FIG. 8 is a timing diagram 300 showing signals V_(IN), V_(DS), V_(FF), V_(REFM), I_(18A), and V_(DR) of converter 10 operating in the critical conduction mode and is similar to FIG. 5, except that it includes a plot of inductor current I_(18A), current information signal, V_(FF), and preset reference voltage V_(REFM). In timing diagram 300, current information signal V_(FF) is higher than preset reference voltage V_(REFM). Accordingly, converter 10 operates in a critical conduction mode. As discussed with reference to FIG. 5, at time t₀, the gate drive signal transitions from logic high voltage level V_(H) to logic low voltage level V_(L). In response to the gate drive signal transitioning to logic low voltage level V_(L), drain-to-source voltage V_(DS) transitions from voltage level V_(ON) to voltage level V_(OFF) and inductor current I_(18A) begins to decrease. At time t₀, drive voltage V_(DR) decreases from logic high voltage level V_(H) to logic low voltage level V_(L), which turns off switching transistor 12. In response to drive signal V_(DR) changing states, drain-to-source voltage V_(DS) begins to increase and inductor current I_(18A) begins to decrease. At time t₁, drain-to-source voltage V_(DS) reaches voltage level V_(OFF). At time t₂, drain-to-source voltage V_(DS) begins to decrease in response to ringing or oscillation currents due to the tank circuit formed between inductor 18A and the parasitic capacitances of converter 10. The parasitic capacitances include the sum of the capacitances attached to the drain of switching transistor 12 and the capacitances associated with the boost diode 36 of the power factor controller. At time t₃, the gate drive signal V_(DR) transitions from logic low level V_(L) to logic high voltage level V_(H), which turns on switching transistor 12. In response to switching transistor 12 being on, drain-to-source voltage V_(DS) transitions to voltage level V_(ON). As discussed with reference to FIG. 5, converter 10 is operating in critical conduction mode, thus there is substantially zero delay (apart from the delay between times time t₂ and time t₃ that may occur while waiting for the next valley) before the gate drive signal is asserted.

It should be noted that between times t₀ and t₄, current indicator voltage V_(FF) and reference voltage V_(REFM) have substantially constant voltage values, where current indicator voltage V_(FF) is greater or higher than preset reference voltage V_(REFM). Thus in critical conduction mode operation, switching transistor 12 turns on without the introduction of a delay period in response to a first falling edge of drain-to-source voltage V_(DS).

FIG. 9 is a timing diagram 302 showing signals V_(IN), V_(DS), V_(FF), V_(REFM), I_(18A), and V_(DR), and V_(SUM) of converter 10 operating in the frequency fold-back mode and may include elements of FIGS. 3 and 6. In timing diagram 302, signal V_(SUM), which is the sum of current information signal V_(FF) and a ramp signal is less than preset reference voltage V_(REFM) from time t₀ to time t₇, greater than preset reference voltage V_(REFM) from time t₇ to time t₁₀, and changes to be less than preset reference voltage V_(REFM) at time t₁₀. It should be noted that the ramp portion of signal V_(SUM) is shifted by signal V_(FF). Accordingly, converter 10 operates in a frequency fold-back mode. At time t₀, the gate drive signal transitions from logic high voltage level V_(H) to logic low voltage level V_(L). In response to the gate drive signal transitioning to logic low voltage level V_(L), drain-to-source voltage V_(DS) transitions from voltage level V_(ON) to voltage level V_(OFF). At time t₁, drain-to-source voltage signal V_(DS) reaches voltage level V_(OFF). At time t₂, drain-to-source voltage V_(DS) begins to oscillate because of a tank circuit formed by inductor 18A and the parasitic capacitances of switching transistor 12. The drain-to-source voltage V_(DS) continues to oscillate between times t₂ and t₁₀. At time t₃, output stage 100 generates a ramp signal that is summed with or added to current information voltage V_(FF). Thus, restart signal V_(SUM) increases from time t₃ to time t₁₀. At time t₇, restart signal V_(SUM) substantially equals preset reference signal V_(REFM). In response to start signal V_(SUM) being substantially equal to preset reference signal V_(REFM), power factor controller 14 is configured to generate a drive signal V_(DR). However, power factor controller 14 includes circuitry that delays the generation of drive signal V_(DR) until enabled by the first negative edge of drain-to-source voltage V_(DS) that occurs after restart signal V_(SUM) is greater than preset reference signal V_(REFM). In the example of FIG. 9, the first negative edge of drain-to-source voltage V_(DS) occurs at time t₉. In response to restart signal V_(SUM) being greater than preset reference voltage V_(REFM) and the occurrence of the negative edge of drain-to-source voltage V_(DS), switching transistor 12 is turned on. Accordingly, at time t₉, drain-to-source voltage V_(DS) decreases to voltage level V_(ON) and inductor current I_(18A) begins to increase. The decrease to voltage level V_(ON) may be delayed to time t₁₀ to wait for the next valley. Thus, in the frequency fold-back mode, power factor controller 14 delays the transition of the gate drive signal from logic low voltage level V_(L) to logic high voltage level V_(H) in accordance with inductor current level I_(18A) and an edge of drain-to-source voltage V_(DS).

FIG. 10 is a timing diagram 304 showing signals V_(IN), V_(DS), V_(FF), V_(REFM), I_(18A), V_(DR), and V_(SUM) of converter 10 operating in the frequency fold-back mode and may include elements of FIGS. 3 and 6. In timing diagram 304, current information signal V_(SUM), i.e., V_(FF)+ramp signal, becomes greater than preset reference voltage V_(REFM) from time t₇ to time t₁₁. Accordingly, converter 10 operates in a frequency fold-back mode. At time t₀, gate drive signal V_(DR) transitions from logic high voltage level V_(H) to logic low voltage level V_(L). In response to the gate drive signal transitioning to logic low voltage level V_(L), drain-to-source voltage V_(DS) transitions from voltage level V_(ON) to voltage level V_(OFF). At time t₂, drain-to-source voltage signal V_(DS) reaches voltage level V_(OFF). At time t₃, drain-to-source voltage V_(DS) begins to oscillate because of a tank circuit formed by inductor 18A and the parasitic capacitances attached to the drain of switching transistor 12 and the capacitance associated with diode 36. The drain-to-source voltage V_(DS) continues to oscillate between times t₃ and t₁₁. At time t₄, output stage 100 generates a ramp signal that is summed with or added to current information voltage V_(FF). Thus, restart signal V_(SUM) increases from time t₄ to time t₁₁. At time t₇, restart signal V_(SUM) substantially equals preset reference signal V_(REFM). In response to start signal V_(SUM) being substantially equal to preset reference signal V_(REFM), power factor controller 14 is configured to generate a drive signal V_(DR). However, power factor controller 14 includes circuitry that delays the generation of drive signal V_(DR) until enabled by a first falling edge of drain-to-source voltage V_(DS) after start signal V_(SUM) is substantially equal to preset reference signal V_(REFM). In the example of FIG. 10, the first negative edge of drain-to-source voltage V_(DS) occurs at time t₁₀. In response to restart signal V_(SUM) being greater than preset reference voltage V_(REFM) and the occurrence of the negative edge of drain-to-source voltage V_(DS), switching transistor 12 is turned on. Accordingly, at time t₁₁, drain-to-source voltage V_(DS) decreases to voltage level V_(ON) and inductor current I_(18A) begins to increase. Thus, in the frequency fold-back mode, power factor controller 14 delays the transition of the gate drive signal from logic low voltage level V_(L) to logic high voltage level V_(H) in accordance with inductor current level I_(18A) and a valley of the ringing or oscillating portion of drain-to-source voltage V_(DS).

It should be noted that in the example shown in FIG. 10, drain-to-source voltage V_(DS) travels through a relative minima and a relative maxima after restart signal V_(SUM) exceeds preset reference signal V_(REFM) but before power factor controller 14 generates drive signal V_(DR). However, in the example shown in FIG. 9, power factor controller 14 generates drive signal V_(DR) after restart signal V_(SUM) exceeds preset reference signal V_(REFM) but before drain-to-source voltage V_(DS) reaches its first relative minima. This occurs because power factor controller 14 is configured to generate drive signal V_(DR) after restart signal V_(SUM) exceeds preset reference V_(REFM) and after the next valley of the ringing or oscillating portion of drain-to-source voltage V_(DS). Because the next valley of the ringing or oscillating portion of drain-to-source voltage V_(DS) occurs sooner in the example of FIG. 9 than FIG. 10, drive signal V_(DR) is generated sooner in the example of FIG. 9 than that of FIG. 10.

FIG. 11 is a block diagram of power factor controller 14 in accordance with an embodiment of the present invention. What is shown in FIG. 11 is a regulator stage 50 and a control stage 150 connected to an output stage 100. It should be noted that regulator stage 50, control stage 150, and output stage 100 may be monolithically integrated together in a semiconductor material to form power factor controller 14. An input of regulator stage 50 is configured to serve as an input terminal 14A of power factor controller 14 and another input of regulator stage 50 is configured to serve as input/output terminal 14B of power factor controller 14; an input of control stage 150 is configured to serve as input terminal 14C of power factor controller 14 and another input of control stage 150 is configured to serve as input/output terminal 14D of power factor controller 14; an output of output stage 150 is configured to serve as output terminal 14E of power factor controller 14; and an input of control stage 150 is configured to serve as input terminal 14F of power factor controller 14.

FIG. 12 is a circuit schematic of regulator stage 50 in accordance with an embodiment of the present invention. Regulator stage 50 includes an operational transconductance amplifier 52 having an inverting input, a noninverting input, and an output. Operational transconductance amplifier 52 is also referred to as an error amplifier. The output of error amplifier 52 is connected to a terminal of an offset voltage source 60, which connection serves as or is connected to input/output terminal 14B of power factor controller 14. Regulator stage 50 further includes a Zener diode 70 coupled to input terminal 14B through offset voltage source 60. A resistor 72 has a terminal connected to the cathode of Zener diode 70 and a terminal connected to a terminal of a resistor 74. The other terminal of resistor 74 is coupled for receiving source of operating potential V_(SS). The terminals of resistors 72 and 74 that are connected together serve as an output 76 at which regulated voltage V_(REGUL) is formed.

FIG. 13 is a circuit schematic of control stage 150 in accordance with an embodiment of the present invention. Control stage 150 may be comprised of an input substage 152 coupled to a dead-time end control substage 156 through a detection substage 154. Input substage 152 may be referred to as a zero current detection stage and detection stage 154 may be referred to as a dead time detection stage. A clock generation or synchronization stage 158 is connected to input substage 152 and ramp substage 156. Input substage 152 comprises a comparator 160 having a noninverting input, an inverting input, and an output. The noninverting input may serve as input terminal 14F of power factor controller 14 and the inverting input may be coupled to a reference voltage V_(REF4). It should be noted that comparator 160 has hysteresis. The output of comparator 160 is commonly connected to an input of a two-input OR gate 162 and to an input of an inverter 164. Input substage 152 further includes an inverter 166 having an output coupled to the input of a delay element 168. The output of delay element 168 is connected to the other input of two-input OR gate 162. A detect latch 170 has an input coupled to the output of two-input OR gate 162. Detect latch 170 and inverter 166 each have an input configured to receive the driver signal from the output of driver 138. The output of detect latch 170 is connected to an input of a two-input AND gate 172 and the output of inverter 164 is connected to the other input of two-input AND gate 172. The output of two-input AND gate 172 serves as the output of input substage 152.

Detection stage 154 may be comprised of a dead-time (DT) detection latch 174 having an input connected to the output of input substage 152, i.e., the output of two-input AND gate 172, and an input configured to receive the driver signal from the output of driver 138, and an output that serves as the output of detection stage 154.

Ramp substage 156 comprises a switch 180 having a current carrying electrode commonly connected to an input of a summer 182, a current source 184 having a terminal coupled for receiving a source of operating potential such as, for example, V_(DD), and a current carrying terminal of a capacitor 186. Switch 180 and capacitor 186 each have a current carrying terminal coupled for receiving a source of operating potential such as, for example, V_(SS). In addition, switch 180 has a control electrode coupled to the output of detection stage 154 through an inverter 178. Summer 182 has an input 183 coupled to capacitor 186, an input 185 configured to receive a signal from input/output terminal 14D of power factor controller 14, and an output 187 that serves as an output of ramp substage 156.

Clock generation stage 158 comprises a comparator 190 having an inverting input coupled to the output of ramp substage 156, a noninverting input coupled for receiving a reference signal V_(REF5), and an output coupled to an input of a two-input AND gate 192. The other input of two-input AND gate 192 is connected to an output of a two-input OR gate 194. The inputs of two-input OR gate 194 are configured to receive a signal from inverter 164 of input substage 152. For example, the output of inverter 164 may be coupled to an input of two-input OR gate 194 through a delay element 196 and to the other input of two-input OR gate 194 through an inverter 198. The output of AND gate 192 is coupled to an input of a two input AND gate 202 through a clocking latch 200. For example, the output of AND gate 192 may be connected to the set input of PWM latch 200 and the reset input of PWM latch 200 may be connected to the output of buffer circuit 138, i.e., to output terminal 14E. One input of two-input AND gate 202 may be commonly connected to the output of inverter 164 and an input of two-input AND gate 172 of input substage 152 and the other input may be connected to the output of clocking latch 200. The output 204 of AND gate 202 serves as the output of clock generation stage 158 and is connected to input 139 of a PWM latch 136.

PWM latch 136 has an input 137 coupled for receiving a PWM signal and an input 139 connected to output terminal 204 of output stage 150. The output of PWM latch 136 is connected to output 14E of power factor controller 14 through a buffer circuit 138.

FIG. 14 is a circuit schematic of a current information generation stage 210 in accordance with an embodiment of the present invention. Current information generation stage 210 may be a portion of control stage 150. Current information generation stage 210 may include a voltage-to-current converter 212 having an input that serves as or may be connected to input terminal 14C of power factor controller 14 and an output connected to an input of a multiplier circuit 216. Current information generation stage 210 further includes a voltage-to-converter 214 that has an input that serves as or may be connected to input/output terminal 14B of power factor controller 14 and an output connected to another input of multiplier circuit 216. Multiplier circuit 216 has an output 219 connected to the inverting input of a comparator 218. The output of multiplier 216 is configured to serve as input/output terminal 14D of power factor controller 14. Comparator 218 has a noninverting input coupled for receiving a reference voltage V_(REF6) and an output configured for transmitting a signal V_(SKIP). Input/output terminal 14D is connected to input 185 of summer 182 illustrated in FIG. 14.

Referring again to FIGS. 11-14, generation of the dead-time by power factor controller 14 will now be described. As discussed above, power factor controller 14 operates in a critical conduction mode in response to current indicator reference signal V_(FF) exceeding a preset reference voltage V_(REFM). Current indicator reference signal V_(FF) is a voltage representation of the level of input current I_(IN). In response to input current I_(IN) being low, power factor controller 14 introduces a dead time in inductor current I_(18A) to reduce the switching frequency of switching transistor 12 (shown in FIG. 1). In accordance with an embodiment, power factor controller 14 linearly reduces the switching frequency of switching transistor 12. Current indicator signal V_(FF) is generated by connecting a resistor 34 between input/output terminal 14D (shown in FIGS. 1, 11, and 14) of power factor controller 14 and a source of operating potential such as, for example, voltage V_(SS). FIGS. 1 and 11 provide a block configuration of power factor controller 14, whereas FIG. 14 provides a more detailed configuration of the circuitry coupled to input/output terminal 14D.

As discussed above, FIG. 14 is a circuit schematic of a current information generation stage 210, which is configured to generate current indicator reference signal V_(FF). In operation, power factor controller 14 is configured to generate a sense voltage V_(SENSE) at input pin 14C. More particularly, input pin 14C is coupled for receiving a portion of the instantaneous input voltage signal V_(IN) through voltage divider network 15 (shown in FIG. 1). It should be noted that voltage V_(SENSE) (shown in FIG. 1) may be used to detect brown-out conditions and the line range. Voltage-to-current converter 212 converts voltage sense signal V_(SENSE) into a current I_(SENSE), which is transmitted to a current multiplier circuit 216. In addition, power factor controller 14 is configured to generate a regulation voltage at pin 14B and to use a voltage-to-current converter 214 to generate a regulation current I_(REG), which is transmitted to current multiplier 216. Multiplier circuit 216 multiplies sense current I_(SENSE) by regulator current I_(REG) and a multiplier gain K_(m) to generate a current signal I_(INF) that is representative of the input current I_(IN). It should be noted that the output signal of multiplier circuit 216 is proportional to a rectified sinusoid whose magnitude is proportional to the power as provided by regulated voltage V_(REGUL), where the rectified sinusoidal shape is provided by voltage sense signal V_(SENSE). It should be further noted that output voltage V_(REGUL) is an output signal of a regulation block, wherein the regulation block regulates the output voltage to a desired level. Current I_(INF) flows through resistor 34 (shown in FIG. 1) to generate current indicator reference signal V_(FF). Input 185 is an input to summer 182 (shown in FIG. 13) and is therefore a high impedance input. Accordingly, current I_(INF) flows through resistor 34 that is coupled to input pin 14D and generates current indicator reference signal V_(FF). Current indicator signal V_(FF) may be referred to as a content signal. Current indicator reference signal V_(FF) may be derived using a signal representative of the power derived from an output signal of regulation block 50 multiplied by a signal representative of the input voltage, wherein regulation block 50 regulates the output voltage to a desired level. It should be understood that current indicator reference signal V_(FF) can be derived from a signal representative of the power.

Power factor controller 14 includes a regulation block or regulator stage 50 illustrated in FIGS. 11 and 12 that is configured to receive a feedback signal V_(FEED) at input pin 14A and to generate a control signal V_(CTRL) at input pin 14B. By way of example, feedback signal V_(FEED) is a portion of a power factor correction output signal. Referring to FIG. 12, the output of operational transconductance amplifier 52 is connected to input pin 14B to provide external loop compensation. Control signal V_(CTRL) is offset by offset voltage v60 of voltage source 60 and reduced by Zener diode 70 and the resistor network comprising resistors 72 and 74 to generate a regulated voltage V_(REGUL) at node 76.

Referring to FIG. 13, a schematic diagram of control stage 150 is illustrated and has been described above. Control stage 150, also referred to as a dead-time detection stage, is configured to generate a clock signal V_(CLK) which is coupled to output stage 100. In operation and referring to FIGS. 1 and 13, a current detection circuit 19 comprising secondary winding or inductor 18B, diode 26, and resistors 28 and 30 are coupled to input pin 14F. Input pin 14F serves as an input pin for zero current detection stage 152 and generates a signal at the output of AND gate 172 in response to detecting current I_(IN) being at a zero level. In accordance with embodiments of the present invention, zero current detection stage 152 detects when the first valley of drain-to-source voltage V_(DS) changes voltage. Reaching the first valley of the drain-to-source voltage V_(DS) may be referred to as reaching dead time. In response to reaching the first valley, AND gate 172 generates an output signal that sets latch 174 of dead-time detection stage 154.

Dead-time detection stage 154 generates an output signal that may be inverted and serves as a control signal for switch 180. In response to being in the dead-time, the control signal from inverter 178 turns off or opens switch 180 and current source 184 sources a current I₁₈₄ that charges capacitor 186. In response to being charged, capacitor 186 generates a ramp signal that is transmitted to input 183 of adder 182. Adder 182 may be referred to as an adder circuit, a summer, a summer circuit, a summing circuit, etc. In addition, current indicator reference signal V_(FF) from input pin 14D (shown in FIGS. 1 and 11) is transmitted to input 185 of adder 182 (shown in FIG. 13). In response to the signals appearing at inputs 183 and 185, summer 182 generates a dead-time control signal V_(SUM) at output 187. Dead-time control signal V_(SUM) may also be referred to as a restart signal.

In response to dead-time control signal V_(SUM) exceeding reference voltage V_(REF5), comparator 190 generates a comparison signal V_(COMP) that can be used to turn on switching transistor 12.

In accordance with another embodiment of the present invention, the turn on of switching transistor 12 is synchronized with the ringing that appears on drain-to-source voltage V_(DS). Synchronization may be accomplished in response to logically ANDing comparison signal V_(COMP) with an indicator signal V_(IND) which indicates that drain-to-source voltage V_(DS) is high. It should be noted that indicator signal V_(IND) is generated in response to signals transmitted from zero current detection input pin 14F through comparator 160, inverters 164 and 198, time delay element 196, and OR gate 194 to an input of AND gate 192. Accordingly, clock generation stage 158 uses comparison signal V_(COMP) and indicator signal V_(IND) to set latch 200. In response to being set, latch 200 generates a signal in a high state at its output Q. It should be noted that latch 200 will be reset by drive signal V_(DR). Thus clock signal V_(CLK) is generated in response to latch 200 being in the high state and the drain-to-source voltage V_(DS) being at a subsequent valley, i.e., the second, third, fourth, etc. valley. It should be noted that delay element 196 is included in case the ringing on the drain-to-source voltage V_(DS) is sufficiently damped that the valleys in the ringing are insufficient to be detectable.

By now it should be appreciated that a converter that includes a power factor controller and a method for operating the converter have been provided. In accordance with embodiments, the converter uses current controlled frequency foldback, wherein the converter operates in critical conduction mode in response to a current information signal being greater than a preset reference signal and in a frequency fold-back mode in response to the current information signal being less than the preset reference signal. In the frequency foldback mode, a current information signal representative of an input current, which is an averaged value of inductor current I_(18A) over a switching period, and independent of a peak input current is created and used to generate a dead-time in the inductor current and thus reduce the switching period. The peak input current can be adjusted without affecting the signal that is representative of the input current and hence without affecting the dead-time. The converter is configured to generate a drive signal in response to the sum of the current information signal and a ramp signal equaling or exceeding the preset reference signal. Generation of the drive signal may be synchronized to a valley of a drain-to-source voltage V_(DS). Thus, the dead-time can be synchronized to the drain-to-source voltage V_(DS) ringing without the occurrence of oscillations between one or more valleys, i.e., without valley jumping. It should be noted that generation of the drive signal may set the duration of the dead-time. The duration of the dead-time may be set in response to a difference between preset reference voltage V_(REFM) and a sum V_(SUM) of a level of the information signal V_(FF) and a ramp signal.

Although specific embodiments have been disclosed herein, it is not intended that the invention be limited to the disclosed embodiments. For example, embodiments for a power factor correction circuit have been described. However, configurations that include other types of converters such as, for example, flyback or forward converters are intended to be included within the scope of the claims. It should be noted than in flyback and forward converters, the operating mode in which a new current cycle occurs in response to the magnetic component being demagnetized is generally referred to as quasi-resonant mode instead of a critical conduction mode. Those skilled in the art will recognize that modifications and variations can be made without departing from the spirit of the invention. It is intended that the invention encompass all such modifications and variations as fall within the scope of the appended claims. 

What is claimed is:
 1. A method for regulating an output voltage, comprising: providing a converter circuit having an input and an output, wherein the converter circuit is coupled to receive a current at the input of the converter circuit and generate a switching signal at the output of the converter circuit; generating an inductor current that flows through a first inductor in response to a load condition on the converter circuit and a switching activity of a switching device; operating the converter circuit in a critical conduction mode in response to a signal representative of power at the input being greater than a first preset reference voltage, wherein, in the critical conduction mode, the inductor current increases from a first level to a second level over a first period of time, decreases from the second level to the first level over a second period of time, increases from the first level to the second level over a third period of time, and decreases from the second level to the first level over a fourth period of time; and generating a dead-time in the inductor current by operating the converter circuit in a frequency fold-back mode in response to the signal representative of power at the input being less than the first preset reference voltage, wherein, in the frequency fold-back mode, the inductor current increases from a third level to a fourth level over a fifth period of time in response to the controller generating a drive signal and the switching device being on, the inductor current decreases from the fourth level to the third level over a sixth period of time in response to the controller generating the drive signal that turns off the switching device, the inductor current remains at the third level over a seventh period of time in response to the controller generating the drive signal that leaves the switching device off, and the inductor current increases from the third level to the fourth level over an eighth period of time in response to the controller generating the drive signal that turns on the switching device.
 2. The method of claim 1, wherein the signal representative of the power at the input is a signal that is representative of a magnitude of the inductor current.
 3. The method of claim 2, further including operating the converter circuit in the critical conduction mode in response to a signal that is representative of a magnitude of the input power being greater than the first preset reference voltage.
 4. The method of claim 1, further including deriving the signal representative of the power at the input using an error amplifier.
 5. The method of claim 1, wherein generating the dead-time in the inductor current includes generating the dead-time in response to a signal that is representative of the magnitude of the input power being lower than the first preset reference voltage.
 6. The method of claim 1, further including increasing the dead-time in response to a decay of a signal representative of the power at the input.
 7. The method of claim 1, further including providing a second preset reference voltage, the first preset reference voltage greater than the second preset reference voltage, and skipping cycles of the inductor current from the input of the converter in response to the signal representative of the power at the input being less than the second preset reference voltage.
 8. The method of claim 1, further including deriving the signal representative of the power at the input using an output signal of a regulation block.
 9. The method of claim 8, wherein the regulation block regulates an output voltage to a desired level.
 10. A method for regulating an output voltage, comprising: providing a converter circuit having an input and an output, wherein the converter circuit is coupled to receive a current at the input, generate a switching signal at the output, and wherein the converter circuit comprises: a regulator stage having first and second inputs and an output; a control stage having first and second inputs and first and second outputs, the first output of the control stage configured to convert a first signal into a content signal that carries information about the first signal, the content signal being a signal that is representative of a magnitude of an inductor current; and an output stage having first and second inputs and an output, the first input of the output stage coupled to the output of the regulator stage and the second input of the output stage coupled to the first output of the control stage; operating the converter circuit in a critical conduction mode in response to the content signal being greater than a first preset reference voltage, wherein in the critical conduction mode the inductor current increases from a first level to a second level over a first period of time, decreases from the second level to the first level over a second period of time, increases from the first level to the second level over a third period of time, and decreases from the second level to the first level over a fourth period of time; and generating a dead-time in the inductor current in response to the content signal by operating the converter circuit in a frequency fold-back mode in response to the content signal being less than the first preset reference voltage, wherein in the frequency fold-back mode the inductor current increases from a third level to a fourth level over a fifth period of time in response to the controller generating a drive signal and the switching device being on, the inductor current decreases from the fourth level to the third level over a sixth period of time in response to the controller generating the drive signal that turns off the switching device, the inductor current remains at the third level over a seventh period of time in response to the controller generating the drive signal that leaves the switching device off, and the inductor current increases from the third level to the fourth level over an eighth period of time in response to the controller generating the drive signal that turns on the switching device.
 11. The method of claim 10, further including increasing the dead-time in response to a decay of the content signal.
 12. The method of claim 10, further including providing a second preset reference voltage, the first preset reference voltage greater than the second preset reference voltage, and skipping cycles of the current from the input of the converter in response to the content signal being less than the second preset reference voltage.
 13. The method of claim 10, further including deriving the content signal representative of the magnitude of the inductor current using an error amplifier. 